An image sensor is essentially formed of an array of pixels comprising “normal” pixels and self-focusing pixels formed in a semiconductor substrate. A self-focusing pixel is a pixel intended to only receive light arriving under a given incidence. Based on pixels receiving light under different incidences, a focusing determination can be performed. Conventional self-focusing pixels comprise screens covering substantially complementary portions of the photodiodes of these pixels, for example, a right-hand portion and a left-hand portion.
FIG. 1 is a cross-section view showing the structure of a self-focusing pixel 11 screened to the left of an image sensor. Pixel 11 comprises an active photodiode area 15 formed in the upper part of a portion of a semiconductor substrate 13. Active area 15 does not entirely cover the substrate portion associated with pixel 11. Indeed, part of the surface is reserved to elements (not shown) for addressing the pixel and reading therefrom.
Substrate 13 is covered with an interconnection network, where metal levels and vias are separated by transparent insulating layers 17. For example, in a first metal level, deposited at the surface of substrate 13, tracks 19 and 21 are formed. Track 19 is arranged so as not to cover active area 15 while track 21 is prolonged by a screen 26 covering the left-hand half of active area 15. Tracks 23 are formed in a second metal level. Vias 25 are formed through insulating layers 17 to connect the two metal levels. Further, in a color sensor, a color filter 27 is arranged above the stack of insulating layers 17, opposite to the portion of substrate 13 associated with pixel 11. Filter 27 is generally covered with an intermediate equalization layer 28.
To concentrate the light intensity received at the surface of pixel 11 towards active area 15, a microlens 29 is arranged at the surface of intermediate layer 28, opposite to the substrate portion associated with pixel 11.
FIG. 2 is a cross-section view showing the structure of a self-focusing pixel 31 screened on the right-hand side of an image sensor. The structure of pixel 31 is identical to that of pixel 11. However, track 21 is arranged so as not to cover active area 15 while track 19 is prolonged by a screen 33 covering the right-hand half of active area 15.
FIG. 3 corresponds to FIG. 1 of French patent No. 2945666 (incorporated by reference). This drawing is a very simplified cross-section view of a square or rectangular image sensor 41 arranged in the focal plane of a lens 43. Sensor 41 is essentially formed of an array of pixels formed in a semiconductor substrate. A pixel 45 and a pixel 47, respectively arranged at the center and at the border of sensor 41, are shown as an example. As illustrated by the light paths shown in full lines and in dotted lines, pixel 45 receives rays centered on an angle of incidence close to 0°, while the pixels placed at the sensor border, and particularly in the corners, such as pixel 47, receive rays centered on a non-zero angle of incidence.
FIG. 4 corresponds to FIG. 3A of French patent No. 2945666. This drawing is a cross-section view showing the structure of a “normal” pixel 51 of image sensor 41. The structure of pixel 51 is identical to the structure of pixels 11 and 31, excluding screens 26 and 33. The shown light path corresponds to the case of a non-zero average angle of incidence, that is, to a pixel located in a peripheral area of the sensor. The focusing point of the microlens for such rays is off-centered with respect to active area 15, which results in a degradation of the sensor sensitivity (vignetting phenomenon). It is desired to address this problem in the prior art. In this patent application, it is accordingly provided to replace conventional lens 31 with an asymmetrical lens. The asymmetrical lens is manufactured so that the received rays converge towards the center of active area 15.
It is desired to more simply manufacture self-focusing pixels and normal pixels located at the border of an array.